One of the wife's powered speakers on her computer is on the fritz, and after seeing some of the other small SE designs (Fleathingie) I thought I'd try something similar as replacements.

I've got some FE103E drivers I'm building accompanying speakers from.

So far the prototype is sounding pretty nice. Volume is adequate for sound reproduction off a computer if you are only listening to occasional CDs etc. In this case, low output power is not going to be a problem.

A couple of minor issues I've encountered so far is the number of LEDs needed to set the bias point near 30mA. I'm purposely limiting the idle current (yes, I know it will increase distortion) so I can use a transformer I already have on hand (Allied 6K49VG).

The transformer is only rated for 50mA, but that is with a capacitive input filter. If I go to a inductor input filter I can increase this 40% to 70mA which will do for this amp. I've already run it for six hours at 70mA load (Plus two op tubes and the driver tube for filament loads) without any significant temp rise (only 23degree F) of the transformer so I think this will be fine.

I'm wondering how well simple shunt screen regulation would work to set the bias point.

I don't know what you are running it at, but the 6N23P seems to be a plate current hog. How about 6N2P or 6N3P at about 3mA? That might lift some of the burden off the PT. A little more heater current though. I've been pleased with the 6N3P-EV as a SE driver on similar B+ as yours, with an IR LED for cathode bias (not regulator fed though).

I missed that. It is currently running a 6N1P as the input tube. I started the schematic from the previous PP amp and must have used the 6N23P phase splitter as the input tube rather than the 6N1P input tube. 6N23P does not sound good at 3ma.

I will probably try a an IXCP10M45 current source in place of the present plate current source to see if I can hear the difference.

I disconnected the Allied 6K49VG transformer / power supply and connected the HP6448B to try it out as I hadn't used it yet.

Power out just kept increasing until I hit Vka = 354V at Ik=61mA and Vg2=275V.

I could see a very slight red fin on the 6P1P-EV with the lights out (14 Cathode LEDs were providing lots of light).

Output voltage into a 8 Ohm resistor was 8vrms with noticeable distortion but no clipping.

8W SE!

I tried upping the voltage further, but it looks like the little AES P-T31 is saturating.

I'll have to wait for my Edcor opts to arrive before I can try to push it further.

Mean time I guess I need to get learn how to measure distortion to get a better idea of how the screen regulator is effecting operation.

When I reduced plate current it looked like distortion was more related to plate current (compression of the curve at lower current) than actually the screen voltage decrease itself.

So my conclusion is that reducing screen voltage does not directly increase distortion, but rather that the associated decrease in bias point is responsible for increased distortion. Therefore as long as I keep the plate current up I can use screen regulation to set my bias point without increasing distortion.

I actually have some 35C5 and 50C5s that I was thinking of building an amp with, but opted for the 6P1P instead as I didn't want to mess with odd filament voltages.

And I'm trying to stick with the 6P1P just so I'm learning on the same tube all the time. At least for now.

I did one other test with the screen regulator. I set the bias at 5 LEDs plus a 33 ohm resistor for about 9.5V, then set the B+ for a plate to cathode voltage of 250V. This resulted in a screen to cathode voltage of 253 (max adjustment). I got 55mA of plate current.

I then cranked the screen down till the plate current dropped to 25mA (too low for good audio production). It was down to 200V, so a 20% change in screen voltage resulted in nearly a 55% decrease in plate current. I need to go back and look at the screen drive plot of the 6P1P to see if I can better understand this relationship.

It looks like it won't take much shift in screen voltage to compensate for non-matched tubes in PP amps if this is used rather than using an adjustable cathode resistor. The MPSA42 is cheap, and the circuit simple so it looks to me like a good option.

I think the 250V limit of the 6P1P is advantageous in that it allows for the use of lower voltage parts (Cheaper) compared to say 6L6, EL34, KT88, etc. We don't get near as much power, but I'm still not convinced I need a lot more.

I think the determining factor will end up being distortion vs normal operating levels. The bigger amps will produce less distortion at the same output level as the smaller amps. The small amps are just working harder.

I disconnected the Allied 6K49VG transformer / power supply and connected the HP6448B to try it out as I hadn't used it yet.

Power out just kept increasing until I hit Vka = 354V at Ik=61mA and Vg2=275V.

I could see a very slight red fin on the 6P1P-EV with the lights out (14 Cathode LEDs were providing lots of light).

Output voltage into a 8 Ohm resistor was 8vrms with noticeable distortion but no clipping.

8W SE!

Did you change your name to/from George

I knew the 6P1P-EVs were tough, but that's over 21W Pd by my calculation.
What brand are your tubes? The ones I have here are SED I think (have a C in a circle with wings).
Push a Chinese 6P1 past 12W and be prepared for trouble.

Parallel SE with MOSFET follower drive might provide interesting results. A solid 10W in PSE at low distortion would be a great result for such inexpensive tubes.

Not really sure it matters, but there already is a DIY tube project called FireFly. It is a tiny guitar amp using 12AU7s in a self-split p-p topology, designed by a guy named Doug Hammond. firefly amp - Google Search